1. Field of the Invention
This invention relates to a method and apparatus for the controlled aeration of stored grain to prevent spoilage and to achieve a desired grain moisture content. More particularly, the invention relates to a method and apparatus for sensing ambient temperature and relative humidity conditions and selectively aerating grain when suitable or best-available ambient temperature and relative humidity conditions are present for achieving stable maintenance of grain moisture content at or near the moisture content level desired by the user and for cooling grain for safer storage.
2. Description of Prior Art
Mold is the major cause of spoilage in stored grain. Mold growth occurs when a moisture and temperature environment suitable for mold is present around the stored kernels. Foreign matter, along with higher temperatures and higher humidities, provide the most favorable environment for mold growth. Clean grain can be stored indefinitely in a storage bin if its moisture and temperature are kept within acceptable limits.
Moisture can be introduced into the air spaces around stored grain (a) by condensation or (b) by the natural respiration of the grain. Condensation can occur when relatively warm, moist air is introduced into the bin and comes into contact with grain that is colder than the air. Condensation more frequently occurs as a result of moisture migration, which happens when natural convection currents within the bin bring warm air from one region of the bin into contact with cooler grain in another region. Crusting and spoiling can result. It is known that the effects of condensation can be minimized by keeping the temperature of the grain at or near the average ambient air temperature.
Natural respiration of stored grain introduces moisture as a function of the temperature and relative humidity of the air surrounding the grain. For a specified temperature and relative humidity combination of the surrounding air, there is a corresponding equilibrium moisture content for the grain; that is, if the air surrounding the grain remains at the specified temperature and relative humidity conditions, the grain will eventually reach the corresponding equilibrium moisture content. Moisture will be given off by the grain kernels when the moisture content of the grain exceeds the equilibrium moisture content supported by the surrounding air conditions; conversely, moisture content of grain will increase when surrounding air conditions will lead to an equilibrium moisture content higher than that present in the grain kernels. In this regard, it should be noted that mold attacks a grain kernel from the outside in; it is the presence of excessive moisture on the outside of the kernel that is to be avoided.
Mold growth on stored grain can thus be restricted by controlling the moisture content and temperature of the grain. The grain temperature and moisture content determine the allowable storage time that the grain can be kept before it spoils. For that reason (and others), grain prices are adjusted for the moisture content of the grain. Grain which has an excessive moisture content must either be dried or used quickly and is therefore of less value than grain marketed at standard moisture content levels.
The effects of condensation can be controlled by maintaining the stored grain at a temperature equal (or nearly equal) to the temperature of the surrounding air. The effects of moisture release due to natural respiration could be avoided by excessively drying the grain. Excessive drying of grain, however, is undesirable for several reasons. First, grain that is at or below its equilibrium moisture content for the ambient air conditions, will not spontaneously give off moisture. It requires energy to remove each additional increment of moisture from a kernel as the kernel dries, and overdrying of the grain below its desired market moisture content consumes energy at an increasingly faster rate as the drying processes, with corresponding higher costs. Secondly, overdrying of grain creates internal stresses within the individual grain kernels cracks and fines, thus lowering the quality of the grain and its market value. Finally, grain is marketed by weight. Overdrying of grain removes more water than is necessary, thereby reducing its total weight. To maximize price, as much moisture should be retained in the kernels as possible, keeping in mind the upper allowable moisture content for safe storage and marketing standards.
Temperature plays an important role in the storage of grain not only because of condensation but because cooler grain, like other refrigerated foodstuffs, lasts longer. Molds grow far more easily at warmer temperatures (e.g., 70 degrees F. and above) than at lower temperatures (e.g., 40 degrees F. and below). Accordingly, subject to condensation problems, keeping grain at cooler temperatures is better.
Proper storage of grain, then, involves several important considerations. First, the temperature of the grain should be as close as possible to the average ambient temperature to avoid moisture migration and condensation. Secondly, the moisture content of the grain should be brought to and kept at a predetermined moisture content level that maximizes the weight of the grain at market time, yet is low enough to be stored safely. Third, grain should be stored at cool temperatures, where possible, and higher grain temperatures should be avoided. Fourth, as a corollary to the first and third considerations, if grain is cooled, its temperature should be lowered in small increments. An additional, economic consideration is that aeration used to achieve or maintain temperature and moisture content should not be performed more than necessary, as extensive aeration fan operation can lead to high energy costs.
U.S. Pat. No. 3,563,460 to Nine discloses a means for controlling the aeration of stored grain. The Nine device incorporates a plurality of temperature sensors located within the grain, and a comparison device for comparing the monitored temperature to a manually set temperature level. An aeration fan is activated when the grain temperature exceeds the set level. The Nine device, however, requires a continual manual adjustment of the set temperature level in order to maintain the grain temperature reasonably near the actual or average ambient temperature. Moreover, the Nine device does not include any mechanism, manual or automatic, to control aeration of the grain as a function of the relative humidity of the ambient air.
U.S. Pat. No. 4,045,878 to Steffen discloses a method for aerating stored grain wherein the stored grain is exposed to a throughput of atmospheric air if the current atmospheric conditions are optimal, in that they are at or near predetermined historical monthly average atmospheric conditions. Although the method disclosed in the Steffen patent, at least in theory, takes into consideration both temperature and relative humidity, application of the method has several drawbacks. First of all, continuous operator monitoring of ambient air conditions is required. Secondly, aeration of grain is premised on historical monthly temperature averages, and not on the actual current average temperature, which can vary considerably from historical seasonal averages. Finally, long periods of time may elapse without any aeration of the grain at all if the predetermined optimal air conditions are not met.
U.S. Pat. No. 4,522,335 to Kallestad et al., assigned to the assignee of the present application, disclosed a method and apparatus for controlled aeration of stored grain to maintain a specified desired grain moisture content. Current ambient temperature and relative humidity conditions are sensed. A running actual average temperature over a specified period is calculated and an equilibrium moisture content for a particular grain type corresponding to the ambient conditions is determined. Aeration is initiated when the current ambient air temperature is within a predetermined acceptable range of the running average ambient temperature and the equilibrium moisture content is within a predetermined acceptable range of the desired grain moisture content. Aeration time is operator-specified at a certain amount per day. If the specified aeration time is not used because ambient conditions are not within acceptable ranges, the unused aeration time is "banked", i.e., stored or backlogged for later use. The acceptable ranges for aeration are expanded in accordance with the increasing amount of time "banked".
U.S. Pat. No. 4,688,332 to Kallestad et al., also assigned to the assignee of the present application, discloses a method and apparatus for controlled aeration of stored grain similar to the method and apparatus of Patent No. 4,522,335. In Patent No. 4,688,332, however, the method and apparatus not only have storage modes but also modes for drying and rewetting grain and certain special and override modes. Also, this patent teaches determining an available amount of aeration time for a specified interval based on the difference between the grain temperature and average ambient temperature.
While useful, the inventions of U.S. Pat. Nos. 4,522,335 and 4,688,332 leave room for improvement. In field experience with these prior devices, it has been noted that, particularly in northerly climates with short falls and very cold winters, grain temperature tends to decrease significantly more slowly than ambient air temperature when such air temperature drops sharply to the freezing point and below. In such a temperature environment the control algorithm that works well for the rest of the year is less than optimal for fall and winter cooling. The lag in grain temperature as compared to ambient air temperature can lead to inadvertent overdrying of grain as it is cooled. It has also been found that while the rate of expansion of the acceptable range of relative humidity used in the prior devices works well for most conditions, in some extreme situations where temperatures are cool and little of the available aeration time has been used, a different rate of expansion can improve results.
A method and device for aeration of stored grain that promotes early cooling of stored grain when fall temperatures begin to drop and that provides more intelligent expansion of the acceptable relative humidity range would be a decided advantage.